Saposnik Beatrice, Lesteven Elodie, Lokajczyk Anna, Esmon Charles T, Aiach Martine, Gandrille Sophie
Inserm U765, Paris, France.
Blood. 2008 Apr 1;111(7):3442-51. doi: 10.1182/blood-2007-08-104968. Epub 2007 Dec 11.
The endothelial cell protein C receptor also exists in soluble form in plasma (sEPCR), resulting from ADAM17 cleavage. Elevated sEPCR levels are observed in subjects carrying the A3 haplotype, which is characterized by a Ser219Gly substitution in the transmembrane domain, rendering the receptor more sensitive to cleavage. Because sEPCR production is not completely blocked by metalloprotease inhibition, we looked for another mechanism. Comparing mRNA expression patterns and levels in A3 and non-A3 cells from 32 human umbilical cord veins, we detected a truncated mRNA in addition to the full-length mRNA. This truncated mRNA was 16 times more abundant in A3 human umbilical vein endothelial cells than in non-A3 human umbilical vein endothelial cells and encoded a protein lacking the transmembrane domain. We stably expressed a recombinant form of this protein (rEPCRisoform) and a protein mimicking the plasma sEPCR (rEPCRsol). Functional studies of the purified recombinant proteins revealed that the rEPCRisoform bound to recombinant protein C with similar affinity than rEPCRsol and that it also inhibited the anticoagulant activity of APC. Trace amounts of the EPCR isoform were found in the plasma of A3 subjects. These results suggest that the sEPCRisoform could contribute to the regulatory effect of sEPCR in plasma.
内皮细胞蛋白C受体在血浆中也以可溶性形式（sEPCR）存在，这是由ADAM17裂解产生的。在携带A3单倍型的受试者中观察到sEPCR水平升高，该单倍型的特征是跨膜结构域中的Ser219Gly替代，使受体对裂解更敏感。由于金属蛋白酶抑制不能完全阻断sEPCR的产生，我们寻找了另一种机制。比较来自32条人脐静脉的A3和非A3细胞中的mRNA表达模式和水平，我们除了检测到全长mRNA外，还检测到一种截短的mRNA。这种截短的mRNA在A3人脐静脉内皮细胞中的丰度比在非A3人脐静脉内皮细胞中高16倍，并编码一种缺乏跨膜结构域的蛋白质。我们稳定表达了这种蛋白质的重组形式（rEPCR异构体）和一种模拟血浆sEPCR的蛋白质（rEPCRsol）。对纯化的重组蛋白的功能研究表明，rEPCR异构体与重组蛋白C结合的亲和力与rEPCRsol相似，并且它也抑制了活化蛋白C的抗凝活性。在A3受试者的血浆中发现了微量的EPCR异构体。这些结果表明，sEPCR异构体可能有助于sEPCR在血浆中的调节作用。
